A computer system that uses Disk Unit Mirroring keeps identical copies of data on a pair of disk units designated as mirrors of one another. If one disk unit in a mirrored pair fails, the system can use the copy of the data on the other unit of that mirrored pair, until the broken unit is repaired and its copy of the data is rebuilt. If the I/O controller for a disk unit fails and the mirror of that unit is attached to a different controller, the system can still continue to run using the disk units attached to the other controller. Likewise, if all of the disk units that are connected to an I/O Processor or Bus are mirrored by disk units connected to different I/O Processors or Busses, the system can survive an I/O Processor or Bus failure and continue to run.
The ability of a mirrored computer system to protect against situations where data could no be accessed, such as when data is destroyed due to a head crash or when data is temporarily unavailable due to a component failure, (hereinafter collectively referred to as "inaccessible data") is directly related to the way the hardware is connected and the way the disk units are arranged in mirrored pairs. Identical hardware can provide either excellent or poor protection.
When a customer or service representative configures a system for mirroring, that person is responsible for arranging the disk units into mirrored pairs. This pairing of the disk units requires an expert understanding of the physical connections of the hardware, so that the hardware path from the disk units of a pair to the system can be duplicated (or mirrored) as much as possible, to increase the level of protection by reducing the number of single hardware failure points. The person pairing up the disk units must also known and follow other system and mirroring restrictions, such as ensuring that the disk units of a mirrored pair are independently serviceable, that like disk device types are paired, etc.
Manual pairing of disk units for mirroring is difficult, time-consuming, and error-prone, and it can frequently result in invalid configurations or configurations with less than optimal pairing and protection.
This problem of manual pairing of disk units for mirroring is magnified whenever the configuration of a computer system changes, such as when a disk unit is added or deleted, when the computer system is recabled, or when a broken unit is replaced with a spare. The level of protection of the computer system against inaccessible data can decrease unbeknownst to the user whenever a configuration change is made. This is clearly undesirable, since the user can have a false sense of security as to the level of protection of his computer system. In order for the user to be adequately protected, he would have to call in his expert to determine if the disk units in his computer system need to be re-paired.